San Sebastian et al1 reported an overall excess for all cancers in San Carlos, Ecuador (standardised incidence ratio (SIR) 2.26; 95% CI 0.97 to
4.46 for males), and an increased mortality due to cancers in males (standardised mortality ratio (SMR) 3.6; 95% CI 1.31 to
7.81). The authors concluded that there was an excess of cancer cases among the villagers linked to environmental pollutants
stemming from oil production activities.

Since clusters present many epidemiological biases, the Centers for Disease Control issued a set of guidelines2 in 1990 to investigate clusters. Under those guidelines, we reviewed the cluster of cancer …